Diamond is a material with semiconductor properties that are superior to silicon (Si), germanium (Ge) or gallium arsenide (GaAs), which are now commonly used. In particular, diamond provides a higher band gap, higher breakdown voltage and greater saturation velocity, which produces a substantial increase in its cutoff frequency and a maximum operating voltage compared to devices fabricated from Si, Ge or GaAs. Furthermore, diamond has the highest of thermal conductivity of any solid at room temperature and excellent conductivity over a temperature range up to and beyond 500.degree. kelvin. The diamond therefore holds potential for efficient semiconductor operation at high frequency and power.
Finally, diamond, by virtue of its small molecular size compared to other materials, provides a small neutron cross section which reduces its degradation rate in radioactive environments. Unfortunately, however, the advantages of diamond as a semiconductor have not been exploited for various reasons.
Although natural diamonds may be of device quality, they are of limited supply and size. Furthermore, most natural diamonds are insulators, and thus introduction, or doping, of electrically active impurities is required to render them useful as semiconductors. Doping by ion implantation has proved to be difficult in diamond. A review of this method may be found in Vavilov, et al "Electronic and Optical Processes in Diamond" copyright in 1975, Nauka, Moscow.
All prior art implantation doping of diamonds known to the present inventor have been high energy ion implantation, such as done routinely with silicon to produce semiconductors which are widely used in commerce. High energy ion implantation, however, causes lattice damage to the crystal. This damage is partially reparable, because the annealing temperature of diamond is above its graphitic conversation point. Complete conversion of the top 20% of the diamond file to graphitic material appears to be unavoidable. Such damage happens even when the implanted atom is smaller than carbon. It is likely that the damage would be much greater with doped ions like sodium or phosphorus, which are larger than carbon.
A second way to produce device quality diamonds is to synthesize them on a desirable substrate by chemical vapor deposition (CVD). In this technique, a gaseous mixture including a carbon supply, usually provided by methane and hydrogen, is pyrolized, or injected into a high frequency plasma, above the substrate surface. Radicals containing carbon react to produce diamond crystals on the substrate, while the hydrogen present is converted to atomic hydrogen, which preferentially etches away graphite and thereby leaves a film which is predominately diamond. This method allows for the possibility of doping by introducing electrically active impurities into the bulk environment above the substrate which are then trapped in the diamond lattices as the lattices are synthesized.
Using the prior art, only boron and nitrogen can be substitutionally grown into the diamond lattice reliably at low pressure. Boron has useful electrical activity, providing p- type material, while nitrogen has been found to be electrically inactive. The prior art has taught some efforts at producing n-type diamond through the introduction of phosphorus and alkali metals. Control over doping levels in the prior art has been very crude because chemical incorporation of impurities appears to be strongly dependant upon growth additions and the dopant feed stocks form long lived volatile residues in the growth chamber.
No prior art known to the present inventor allows the reliable production of both n and p type diamond doped with any desirable material without substantial lattice damage.